In commonly owned U.S. Pat. No. 3,692,964, issued in the names of Roberto Camiciottoli and Maurizio Palombari, there has been disclosed a remote-testing arrangement for the repeaters of such a PCM channel by which an identification code individual to a selected repeater is sent out from a PCM terminal at one end of the channel and is picked up by the repeater for which it is intended, the repeater being for this purpose equipped with a decoder which controls the closure of a loop over the incoming and outgoing signal paths of the channel upon recognizing the assigned code. This code is then sent back over the loop, as a test signal, to the originating terminal in order to facilitate an evaluation of the repeater concerned.
Such a remote-testing arrangement, though satisfactory in principle, is relatively complex and includes energy-dissipating components such as shift registers so that its rate of power consumption differs but little from that of the supervised telecommunication system itself. Safety regulations in some countries require that currents transmitted over longdistance lines be limited to a few tens of milliamperes, e.g. 40 mA; this limit is somewhat difficult to observe in high-speed switching systems in which semiconductors are turned on and off at rates corresponding to the usual bit frequency of about 2 Mbit/sec. Furthermore, the need for differentiating between test codes assigned to the various repeaters of a channel restricts the choice of available code signals and reduces the flexibility of the system inasmuch as such test codes must have certain characteristics distinguishing them from the normally transmitted message codes.